1. Field of the Invention
This invention relates to a dry type toner for electrophotography suitable for producing color images of high quality.
2. Discussion of Background
In a dry-type electrophotographic process, latent electrostatic images formed on a photoconductor are developed with a dry-type toner. The resulting toner images are transferred to a transfer sheet or copy paper, and thermally fixed thereto, for instance, by a thermal image fixing roller.
Dry-type toners for use in the above process comprise a binder resin and a coloring agent as the main components, and when necessary, auxiliary components such as a charge controlling agent and an off-set preventing agent are added thereto. With the necessary properties for the toners, such as transparency, insulating property, water-resistance, fluidity as powder, mechanical strength, glossiness, crushability and thermoplasticity, taken into consideration, polystyrene resin, styrene--acrylic acid copolymer, polyester resin and epoxy resin are in general use as binder resins of the dry-type toners. Of these binder resins, polystyrene resin is most widely employed because of its excellent crushability, water-resistance and fluidity.
Polystyrene resin, however, is readily plasticized by plasticizers. Therefore, when a copy paper which bears toner images reproduced by a toner containing polystyrene resin is placed in a folder made of a polyvinyl chloride sheet in close contact therewith for a while, the polystyrene resin contained in the toner is plasticized by a plasticizer contained in the polyvinyl chloride sheet, so that when the copy paper is separated from the folder, the fixed toner images are partially or totally transferred to the surface of the folder, staining both the folder and the copy paper. The above problem is also caused when images are reproduced by a toner containing polyester resin as the binder resin.
In order to prevent such transfer of toner images to a sheet of polyvinyl chloride, it has been proposed to blend epoxy resin, which is not plasticized by an ordinary plasticizer employed in polyvinyl chloride, with the polystyrene resin or polyester resin as disclosed in Japanese Laid-Open Patent Applications 60-263951 and 61-240252.
However, when toners comprising a mixed binder resin consisting of different types of resins as mentioned above are employed for the production of images, problems such as the so-called off-set phenomenon and the curling of toner-image-bearing copy paper are caused by the incompatibility between the different resins used in the binder resin. In addition, some other problems occur with the glossiness of toner images, and the coloring performance and transparency of the toners.
These problems cannot be completely solved even by use of conventional modified epoxy resins, for instance, acetylated epoxy resin which is disclosed in Japanese Laid-Open Patent Application 61-235852.
Further, in recent dry-type electrophotography, rollers coated with silicone rubber o with highly durable Teflon are employed as thermal image fixing rollers. In general, a Teflon-coated roller has a relatively rough and hard surface, so that there is a demand for a toner with improved image-fixing performance which is capable of forming glossy toner images, without causing the problems of the off-set of toner images and the curling of toner-image-bearing copy sheets.
Furthermore, when an epoxy resin is used alone, it has the following problems in addition to the aforementioned problems:
(1) Epoxy resin readily reacts with amines to form a hardened epoxy resin with a cross-linked structure. Therefore, when kneading epoxy resin with, for example, dyes, pigments and charge controlling agents of an amine type, there is the risk that the epoxy resin is hardened and the resulting mixture cannot be used as a dry-type toner for electrophotography.
(2) The epoxy groups contained in epoxy resin are biochemically active and stimulative to the human skin, so that great care must be taken when handling epoxy resin.
(3) The epoxy groups of epoxy resin are hydrophilic, so that the epoxy resin absorbs water under the conditions of high temperatures and high humidities. When the epoxy resin absorbs water, the chargeability of a toner which contains such an epoxy resin is decreased and the toner is deposited on the background of copy paper, and it becomes difficult to clean a toner-deposited photoconductor.
It is a common practice to use epoxy resin together with a coloring agent and a charge controlling agent in a dry-type toner for electrophotography. However, when preparing the toner, it is quite difficult to disperse a coloring agent and a charge controlling agent homogeneously in an epoxy binder resin. When the coloring agent is not well dispersed, the color of toner images becomes dull and the color density thereof decreases; and when the charge controlling agent is not well dispersed, the toner particles are not uniformly charged, which brings about problems such as toner deposition on the background of copy sheets, the scattering of toner particles, the decrease of image density, the formation of unclear-cut images, and difficult cleaning of the photoconductor.
Japanese Laid-Open Patent Application 61-219051 discloses a toner which contains as a binder an ester-modified epoxy resin with .crclbar.-caprolactone. This ester-modified epoxy resin can improve the plasticizer-resistance and fluidity of the toner. However, in this modification, the epoxy resin is esterified in a ratio of 3 wt. % to as high as 90 wt. %, so that the softening point of the resin becomes excessively low. By using a toner containing such an ester-modified resin, glossy images can never be produced.
Japanese Laid-Open Patent Application 52-86334 discloses a toner having positive chargeability, which is prepared by reacting a primary or secondary aliphatic amine with terminal epoxy groups contained in an epoxy resin. However, as mentioned previously, the epoxy groups and an amine compound tend to react to form a cross-linked epoxy resin, so that there is the risk that the resin becomes unusable for the toner. Further, by the above reaction, it is difficult to control the charging-level of the resin as desired, although the resin can be charged to a positive polarity.
Japanese Laid-Open Patent Application 52-156632 discloses that one or both of the terminal epoxy groups of an epoxy resin are reacted with alcohol, phenol, a Grignard reagent, sodium organic acid acetylide or alkyl chloride. However, if the resulting compound contains an unreacted epoxy group, the properties of the epoxy resin, such as reactivity with amines, toxicity, and hydrophilic property, become problems again as mentioned previously. In addition, some of the above compounds which are employed for the reaction with the epoxy resin are hydrophilic, and have adverse effects on the chargeability and crushability of the toner. Therefore, it can be said that these compounds are not useful in practice.